The main objective of this project is the study of magnesium sulfate capability for inducing controlled hypotension for reducing blood loss during posterior fusion of spine and comparing that with the effects of remifentanil. The study designed as a prospective randomized double blind clinical trial in two groups (magnesium & remifentanil). The inclusion criteria includes: patients with 20- 60 years old, and ASA I & II physical status in candidates for posterior fusion of lumbar spine. Patients with the history of systemic cardiovascular, hepatic and renal diseases as well as those with allergic reaction to magnesium and remifentanil were excluded. Then 40 patients were allocated randomly into two equal groups. After induction of anesthesia and placing the patients in prone position and making sure that they are hemodynamically stable, in group I, a loading dose of magnesium (50 mg/kg) was administered in 10 min and then a 15mg/kg were infused continually during operation. And in the second group, infusion of remifentanil 0.1- 0.2µg/kg/min instead of magnesium sulfate was used. The remaining anesthetic drugs and techniques are the same for both groups. The aim of this intervention is the reduction of mean arterial pressure to 60-75 mmHg. If the success could not be achieved in 10 min, infusion of TNG will be started and documented for comparison between groups. In addition, amount of blood loss, total fluid therapy, and need for transfusion, urine output, satisfaction of surgeons from the surgical field, and hemodynamic variables will be recorded. Complications such as arrhythmias, bradycardia, hypotension and therapeutic intervention for them also shall be recorded and compared between two groups.